PARIS - A long-awaited communications window between Earth and robot lab Philae, zipping through space perched on a comet, opened Thursday without immediate signs of life, the European Space Agency (ESA) said.
The systems for comet orbiter Rosetta to talk to the fridge-sized probe were switched on at 0100 GMT, but the first optimum slot for signal transmission came and went without word.
"No signal has been received yet", an ESA spokesman told AFP.
The communications line will remain open until March 20, with 11 optimum slots for signal transmission in total - roughly two periods per day when the orbiter, lander and Sun are best aligned for contact.
The 100-kilogramme (220-pound) probe Philae landed on Comet 67P/Churyumov-Gerasimenko on November 12 after a 10-year trek piggybacking on its mother ship, Rosetta.
But the landing did not go smoothly. Philae bounced twice off the comet's hard surface before settling at an angle in the shadow of a cliff.
Philae had enough stored power in a battery for 60 hours of experiments. It sent home reams of precious data before going into standby mode.
As comet "67P" draws closer to the Sun, it is hoped better light conditions will recharge Philae's solar-powered batteries and it will wake up.
But the probe's exact position is not known, nor whether it can be reached by the Sun's rays.
For this week, "the idea is to see if we can receive a signal from the lander," Rosetta Mission Manager Patric Martin told AFP by telephone.
"But the difficulty is whether it has enough energy first to wake up and then to communicate with Rosetta. We are just trying, and we will try again if it doesn't work this week.
"We will have more opportunities in the coming weeks and months," said Martin - adding that the likeliest chance may actually be around May, June or July.
The comet is expected to reach its closest point to the Sun, at a distance of some 186 million kilometres (116 million miles), on August 13.
Rosetta's journey to the comet took more than a decade from 2004, and 6.5 billion kilometres (four billion miles).
Having placed Philae on the dusty iceball's surface, Rosetta continues to orbit 67P as it loops around the Sun.